In memory circuits a number of desirable characteristics, such as small chip size, reliability and speed may have detrimental effects on each other. In ROMs virtual ground techniques have been used to reduce chip size and improve speed. In a virtual ground system, two adjoining ROM cells share a source connection which is selectively switched to ground potential when one of the two ROM cells is selected. A characteristic of virtual ground systems, however, is a limitation on the voltage swing on the bit line. A bit line precharged to 4.0 volts, for example, may be allowed to drop to no lower than 3.0 volts in virtual ground system. The lower allowed limit is at least somewhat affected by the threshold voltage of the transistors used and word line voltage.
A maximum voltage swing of 1.0 volts is typically sufficient for reliable detection of the state of a selected cell because a typical cell has only two states. In reducing chip size by using four state cells instead of the typical two, the available 1.0 volt swing must be used to detect all four states. In such a case it becomes more important to preserve as much of the available voltage differential as possible. Voltage swing is also often minimized to increase speed as well. As the available voltage swing is reduced, the importance of preserving voltage differential for reliable sensing increases.